Laser illumination system with reduced speckle

ABSTRACT

A despeckling device and method in which an optical path difference staircase element is disposed between a fly&#39;s eye lens array and the image plane in a position near the focus position of the fly&#39;s eye lens array, and a laser generating unit generates and transmits pulsed laser beams to the optical path difference staircase element, wherein the pulsed laser beams are driven at a very short pulsed rate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to and claims the benefit of provisional U.S. application No. 61/388,238 filed Sep. 30, 2010.

FIELD OF THE INVENTION

This invention relates to a laser annealing device with reduced speckle, especially to a laser annealing machine to create a thin film transistor for a large size organic LED display. This invention also relates to laser display devices and laser display methods, and more particularly, to laser display devices and laser display methods directed to achieving reduced speckle.

RELATED ART

In general, a laser is an optical source that emits a coherent light beam (also referred to herein as “coherent light” or “laser light”). The coherent light may be emitted as a relatively narrow beam and may be focused to very small spots. Because lasers emit coherent light, lasers may be prone to speckle. Speckle is a random intensity pattern on reflection from a diffuse surface generally caused by mutual interference of multiple laser beams from a coherent source reflected from different reflection points. For example, a coherent light beam may be scattered at a rough surface (e.g., a piece of paper, a display screen or a metallic surface). Coherent light scattered by the rough surface can exhibit variations in optical paths between any of two different raised areas on the surface, to produce an interference (speckle) pattern if the optical path is relatively shorter than the coherence length of the laser source. The speckle pattern is typically observed as a random granular pattern. Speckle patterns may severely degrade the image quality of components illuminated with a laser source, such as laser annealing, laser projection displays and laser microscopes. Accordingly, it is desirable to reduce or eliminate speckle from a laser source.

For light of high or moderate coherence, conventional speckle reduction techniques typically involve generating many independent speckle patterns that may average each other out at the image plane. In general, speckle reduction methods may be categorized as belonging to one of dynamic reduction methods and static reduction methods. Dynamic reduction methods typically involve the use of a time-varying component. For example, vibration of a laser fiber or screen, rotation of a diffuser, random shuttering of a light valve, and variation of polarization states with time. Static reduction methods typically involve the use of stationary components. For example, static methods may employ stationary diffusers and stationary optical path difference elements such as an optical fiber bundle in which each of the individual optical fibers have different lengths.

Dynamic reduction methods typically outperform static reduction methods, because the dynamic reduction methods are able to more effectively average the speckle patterns. However, devices that use dynamic reduction methods tend to be larger than those devices that use static reduction methods, for example, because of the number of additional mechanical components involved to generate the time-varying component. Devices that use static reduction methods may also be large. For example, devices that include static optical path difference elements such as in an optical fiber may use large path lengths to produce a substantial optical path difference.

Another technique that may be used to produce a homogenously-illuminated field includes the use of microlens arrays, by splitting the incident laser beam into a number of beamlets, depending on the number of microlenses. Microlens arrays, however, tend to give rise to speckle because in forming the ultimate image they merge the split beamlets.

A conventional despeckling method according to prior art document JPA2008-159348 is shown in FIG. 1. As shown in FIG. 1, a staircase element 1 is disposed between a laser diode 2 and a first fly's eye lens array (not shown) on an optical path. That is, in the direction of travel of the laser beam toward the image plane, the staircase element is disposed after the laser diode and before the first fly's eye lens array. Furthermore, the staircase element is configured to make a path difference for each beamlet 4 generated from the received laser beam 3. These beamlets 4 are then output from the staircase element 1 to the first fly's eye lens array. The first fly's eye lens array then focuses the beamlets 4 onto approximately the first surface of the field lens to display an image. By creating a path difference between each of the beamlets, the staircase element 1 reduces interference between the beamlets, thereby reducing speckle.

However, the conventional despeckling method shown in FIG. 1 has the following drawbacks.

First, because the incident beam 3 passes through the staircase element 1 and gets diffracted before entering the first fly's eye lens, the diffracted beamlets 4 may deviate in uniformity before entering the first fly's eye lens, thereby reducing the effectiveness of the despeckling.

Second, each of the beamlets 4 diffracted by the staircase element 1 can enter not only the respective targeted lenslet, but also an adjacent lenslet, thereby further reducing the effectiveness of homogenization by using the fly's eye system.

SUMMARY OF THE INVENTION

Aspects of certain embodiments of the present invention solve these and/or other problems associated with the related art by providing an improved despeckling device and method in which an optical path difference staircase element is disposed between the first fly's eye lens array and the image plane in a position near the focus position of the first fly's eye lens array, and a laser generating unit generates and transmits pulsed laser beams to the optical path difference staircase element, wherein the pulsed laser beams are driven at a very short pulsed rate.

According to an aspect, there is provided an a despeckle unit, comprising a first transparent element comprising a plurality of microlenses to receive collimated light having a coherence length and to output a beamlet from each of the microlenses; and a second transparent element comprising a plurality of steps having a one-to-one correspondence with the plurality of microlenses, wherein each of the steps receives one of the beamlets and outputs the beamlet to an image plane, where a height of each step of at least two of the steps is configured to produce an optical path difference of the beamlet longer than the coherence length, wherein the second transparent element is disposed approximately at a foci of the beamlets output from the first transparent element. The collimated light may comprise pulsed laser beams driven by a pulse of less than 10 nanoseconds. The pulse may reduce a coherence of the laser beam and the steps may be configured to further reduce the coherence of the laser beam not reduced by the pulse, to substantially despeckle the pulsed laser beam. At least two of the steps may be configured as a one-dimensional staircase and the microlenses are configured as a one-dimensional array of microlenses. The collimated light may be linearly polarized with a polarization direction, and the second transparent element may comprise at least one physical step comprising an optical wave plate disposed on a first portion of the at least one physical step which is configured to change the polarization direction of the collimated light, and a second portion of the at least one physical step which does not include the optical wave plate; and the optical waveplate and the second portion may each comprise one of the steps having the one-to-one correspondence with the microlenses. Alternatively, the collimated light may be linearly polarized with a polarization direction, and the second transparent element may comprise at least one physical step comprising a first optical wave plate disposed on a first portion of the at least one physical step which is configured to change the linearly polarized light to right circular polarized light, and a second optical wave plated disposed on a second portion of the at least one physical step which is configured to change the linearly polarized light to left circular polarized light; and the first optical wave plate and the second optical wave plate each comprise one of the steps having the one-to-one correspondence with the microlenses.

According to another aspect, there is provided a despeckle unit, comprising a first transparent element comprising a plurality of microlenses to receive collimated light having a coherence length and to output a beamlet from each of the microlenses; and a second transparent element comprising a plurality of steps having a one-to-one correspondence with the plurality of microlenses, wherein each of the steps receives one of the beamlets and outputs the beamlet to an image plane, wherein a height of each step of at least two of the steps is configured to produce an optical path difference of the collimated light longer than the coherence length, wherein the second transparent element is disposed in a location relative to the first transparent element such that edges of the second transparent element parallel to an optical path of the beamlets exiting the second transparent element do not diffract the beamlets. The collimated light may comprise pulsed laser beams driven by a pulse of less than 10 nanoseconds.

According to another aspect, there is provided a despeckling laser unit to despeckle a laser beam, comprising a laser generating unit to generate a pulsed laser beam having a coherence length; a first transparent element comprising a plurality of microlenses to receive the pulsed laser beam and to output a beamlet from each of the microlenses; and a second transparent element comprising a plurality of steps corresponding to the plurality of microlenses, wherein each of the steps receives one of the beamlets and outputs the beamlet to an image plane, wherein a height of each step of at least two of the steps is configured to produce an optical path difference of the pulsed laser beam longer than the coherence length, wherein the second transparent element is disposed approximately at the foci of the beamlets output from the first transparent element. The pulsed laser beams may be driven by a pulse of less than 10 nanoseconds. The despeckling laser unit may further comprise a collimator disposed between the laser generating unit and the first transparent element to receive the pulsed laser beam and output a collimated laser beam; and a field lens to receive the beamlets output from the second transparent element and focus the received beamlets on the image plane. The at least two of the steps may be configured as a one-dimensional staircase and the microlenses are configured as a one-dimensional array of microlenses. The pulsed laser beam may be linearly polarized with a polarization direction; and the second transparent element may comprise at least one physical step comprising an optical wave plate disposed on a first portion of the at least one physical step which is configured to change the polarization direction of the pulsed laser beam, and a second portion of the at least one physical step which does not include the optical wave plate; and the optical wave plate and the second portion each comprise one of the steps having the one-to-one correspondence with the microlenses. Alternatively, the pulsed laser beam may be linearly polarized with a polarization direction; and the second transparent element may comprise at least one physical step comprising a first optical wave plate disposed on a first portion of the at least one physical step which is configured to change the linearly polarized light to right circular polarized light, and a second optical wave plated disposed on a second portion of the at least one physical step which is configured to change the linearly polarized light to left circular polarized light; and the first optical wave plate and the second optical wave plate each comprise one of the steps having the one-to-one correspondence with the microlenses. The despeckling laser unit may further comprise a third transparent element comprising another plurality of microlenses disposed after the second transparent element and corresponding to the plurality of microlenses, to provide focus control of the beamlets.

According to another aspect, there is provided a despeckling laser array, comprising a plurality of the despeckling laser units described above and a field lens to focus the beamlets output from each of the plurality of despeckling laser units onto the image plane.

According to another aspect, there is provided a despeckling laser assembly, comprising a despeckling laser array as described above; a base plate to support the despeckling laser array; a circuit board attached to one end of the despeckling laser array; and at least one driver integrated circuit mounted on the circuit board to drive the laser generating units of the despeckling laser array.

According to another aspect, there is provided an annealing system to anneal a substrate, comprising a plurality of the despeckling laser assemblies described above, disposed above a front surface of the substrate, such that each of the despeckling laser assemblies is configured to focus the beamlets on the substrate, wherein each of the despeckling laser assemblies is movable to enable the annealing system to anneal the front surface of the substrate. In this annealing system, the substrate may comprise amorphous silicon for organic LED displays.

According to another aspect, there is provided a one-dimensional crossed despeckling unit, comprising a first transparent element comprising a first surface having a first plurality of first microlenses to receive collimated light having a coherence length, and output a first plurality of first beamlets corresponding to the first plurality of microlenses, and a second surface having a second plurality of second microlenses to receive the collimated light, and output a second plurality of second beamlets corresponding to the second plurality of microlenses; and a second transparent element comprising a first plurality of first steps oriented such that at least one of the first steps corresponds to at least one of the first beamlets; and a second plurality of second steps oriented such that at least one of the second steps corresponds to at least one of the second beamlets, wherein a height of each step of at least two steps from among the first steps and the second steps is configured to produce an optical path difference of the pulsed laser beam longer than the coherence length, and the second transparent element is disposed approximately at a foci of the first beamlets and the second beamlets output from the first transparent element. The collimated light may comprise pulsed laser beams driven by a pulse of less than 10 nanoseconds. The pulse may reduce a coherence of the laser beam and the steps may be configured to further reduce the coherence of the laser beam not reduced by the pulse, to substantially despeckle the pulsed laser beam. The first transparent element may comprise a one-dimensional crossed microlens array, and the second transparent element may comprise a combination of a first staircase element having the first plurality of steps and a second staircase element having the second plurality of steps. The first steps may each have the same first height, the second steps may each have the same second height, with the first height being different from the second height. The first staircase may be provided in plural.

According to another aspect, there is provided a laser module comprising a housing; a plurality of laser diodes disposed at one end of the housing to generate respective pulsed laser beams having respective coherence lengths; a first transparent element, disposed after the plurality of laser diodes in the direction of travel of the laser beams, comprising a plurality of microlenses to receive the pulsed laser beams and to output a beamlet from each of the microlenses; and the one-dimensional crossed despeckling unit, as described above, disposed after the first transparent element in the direction of travel of the laser beams, wherein each of the first staircases corresponds to at least one of the laser diodes. The laser diodes may be arranged in an M×N grid, where M an N are positive integers respectively representing a number of laser diodes in columns and rows of the grid, with the plurality of first staircases comprising M first staircases, and each one of the M first staircases corresponding to a respective one of the M columns. In one aspect, M>N, N≧1, M≧2; a beam shaping axis is arranged in a direction corresponding to the N laser diodes; and each one of the M×N laser diodes generates approximately 0.2 watts (W) of output power.

According to another aspect, there is provided a method to despeckle a laser beam, comprising generating a pulsed laser beam having a coherence length; transmitting the pulsed laser beam through a first transparent element comprising a plurality of microlenses so that the pulsed laser beam is output as a beamlet from each of the microlenses; and transmitting each one of the beamlets through a respective step included in a second transparent element comprising a plurality of the steps having a one-to-one correspondence with the plurality of microlenses, to an image plane, wherein a height of each step of at least two of the steps is configured to produce an optical path difference of the beamlets longer than the coherence length, wherein the second transparent element is disposed approximately at the foci of the beamlets output from the first transparent element. The generating of the pulsed laser beam may comprise driving the pulsed laser beam using a pulse of less than 10 nanoseconds. The method may further comprise collimating the generated pulsed laser beam and outputting the collimated generated pulsed laser beam to the first transparent element; and focusing the received beamlets transmitted through the respective steps by the second transparent element on the image plane using a field lens. The at least two of the steps may be configured as a one-dimensional staircase and the microlenses may be configured as a one-dimensional array of microlenses. The method may further comprise polarizing the pulsed laser beam with a linear polarization having a polarization direction; and changing the polarization direction of the pulsed laser beam by passing the pulsed laser beam through an optical wave plate comprising one of the steps of the second transparent element. The method may further comprise polarizing the pulsed laser beam with a linear polarization; and changing the linear polarization of the pulsed laser beam to right and left circular polarization by passing the pulsed laser beam through corresponding first and second optical wave plates each comprising one of the steps of the second transparent element. The method may further comprise transmitting each one of the beamlets output from the second transparent element through a third transparent element comprising another plurality of microlenses, to provide focus control of the beamlets.

According to another aspect, there is provided a two-dimensional despeckling unit, comprising a first transparent element comprising a surface having a plurality of microlenses to receive collimated light having a coherence length from a pulsed laser beam, each of the microlenses configured to output a beamlet which is shaped in two-dimensions; and a second transparent element comprising a light incident surface forming a two-dimensional area comprising two first boundaries and two second boundaries perpendicular to and connecting the two first boundaries; and a plurality of steps protruding out from the light incident surface and arranged in rows, wherein the steps in each row are configured to increase in height along a first direction parallel to the first boundaries, and the rows increase in height along a second direction parallel to the second boundaries, each of the steps having a different height from each other, and each of the steps being configured to receive a corresponding one of the beamlets; wherein the height of each step is configured to produce an optical path difference longer than the coherence length, and the light incident surface is disposed approximately at a foci of the beamlets output from the first transparent element.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred non-limiting examples of exemplary embodiments of the invention, and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles and concepts of the invention, in which like reference characters designate like or corresponding parts throughout the several drawings. Preferred embodiments of the present invention will now be further described in the following paragraphs of the specification and may be better understood when read in conjunction with the attached drawings, in which:

FIG. 1 depicts a conventional despeckling method;

FIG. 2( a) depicts a cross-section diagram of a laser beam homogenizer according to a first embodiment of the present invention, with respect to a fast axis;

FIG. 2( b) depicts a cross-section diagram of the laser beam homogenizer shown in FIG. 2( a) with respect to a slow axis;

FIG. 3 depicts a magnified view of the despeckle unit shown in FIGS. 2( a) and 2(b);

FIGS. 4( a), 4(b), 4(c) and 4(d) depict a comparison of a power spectrum and visibility in continuous wave(CW) or non-short pulse operation (FIGS. 4( a) and 4(b)) and in short pulse operation according to embodiments of the present invention (FIGS. 4( c) and 4(d));

FIG. 5 depicts a laser beam homogenizer according to a second embodiment of the present invention;

FIG. 6 depicts a despeckle unit according to a third embodiment of the present invention;

FIG. 7 depicts a despeckle unit according to a fourth embodiment of the present invention;

FIG. 8( a) depicts a cross-section diagram of array according to a fifth embodiment of the present invention, with respect to non-shaping axis;

FIG. 8( b) depicts a cross-section diagram of the array shown in FIG. 8( a) with respect to beam shaping axis;

FIG. 9( a) depicts a side-plan view diagram of unit according to a sixth embodiment of the present invention, with respect to the x and y axes;

FIG. 9( b) depicts the unit shown in FIG. 9( a) with respect to the x and z axes;

FIG. 9( c) depicts the unit shown in FIG. 9( a) with respect to the y and z axes;

FIG. 10 depicts a top-plan view diagram of a system 1000 according to a seventh embodiment of the present invention;

FIG. 11( a) depicts a one-dimensional crossed staircase element according to an eighth embodiment of the present invention;

FIGS. 11( b) and 11(c) depict the one-dimensional crossed staircase element shown in FIG. 11( a) in use, viewed from a top perspective and a side perspective, respectively;

FIG. 12 depicts a one-dimensional crossed despeckling array 1300 according to a ninth embodiment of the present invention;

FIG. 13 depicts a 4×2 laser diode (LD) module for laser displays according to a tenth embodiment of the present invention;

FIG. 14 depicts a two-dimensional staircase element according to an eleventh embodiment of the present invention along with a projected plane representing the total step height of each beamlet passing through the two-dimensional staircase element; and

FIGS. 15( a) and 15(b) depict the two-dimensional staircase element shown in FIG. 14 in use, viewed from a top perspective and a side perspective, respectively.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the presently non-limiting, exemplary and preferred embodiments of the invention as illustrated in the accompanying drawings. The nature, concepts, objectives and advantages of the present invention will become more apparent to those skilled in the art after considering the following detailed description in connection with the accompanying drawings. The following description is provided in order to explain preferred embodiments of the present invention, with the particular features and details shown therein being by way of non-limiting illustrative examples of various embodiments of the present invention. The particular features and details are presented with the goal of providing what is believed to be the most useful and readily understood description of the principles and conceptual versions of the present invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for the fundamental understanding of the present invention. The detailed description considered with the appended drawings are intended to make apparent to those skilled in the art how the several forms of the present invention may be embodied in practice.

FIGS. 2( a) and 2(b) depict cross-sectional diagrams of an exemplary laser beam homogenizer 100 (also referred to herein as homogenizer 100), according to a first embodiment of the present invention. In particular, FIG. 2( a) is a cross-section diagram of homogenizer 100 with respect to a fast axis of laser source 102; and FIG. 2( b) is a cross-section diagram of homogenizer 100 with respect to a slow axis of laser source 102. FIG. 3 is a magnified view of the despeckle elements or despeckle unit 106 shown in FIGS. 2( a) and 2(b).

Homogenizer 100 includes short pulse laser driver 101, laser source 102, collimator 104, despeckle elements 106 and field lens 108. In operation, laser source 102 emits coherent light beam 116. Collimator 104 collimates coherent light beam 116 received from laser source 102, to form collimated light beam 118. It is understood that collimator 104 may collimate coherent light beam 116 to form collimated light (collimated light with no divergence) or approximately collimated light (collimated light with some degree of divergence or convergence).

Despeckle elements 106 include a microlens array 114 (also known as a fly's eye lens array) and a staircase element 112 formed of a transparent material. The microlens array 114 receives the collimated light beam 118 from the collimator 104 and splits the collimated light beam 118 into beamlets 120. More specifically, the microlens array 114 includes a number of microlenses and splits the collimated light beam 118 into a number of beamlets corresponding to the number of microlenses. As shown in FIGS. 2( a) and 3, the microlens array 114 includes three microlenses 114-1, 114-2, and 114-3 (FIG. 3) and splits the collimated light beam 118 into three corresponding beamlets 120-1, 120-2, and 120-3, although it is understood by those skilled in the art that the microlens array 114 may have more than or less than three microlenses. The beamlets 120-1, 120-2 and 120-3 are then transmitted to the staircase element 112, which functions as an optical path difference element. By creating a path difference between each of the beamlets 120, the staircase element 112 reduces the coherence of the beamlets 120 as compared to the coherent light beam 116. The beamlets 120 are then superimposed on an image plane 110 by a field lens 108, to produce a homogenously illuminated field with remarkably reduced or eliminated speckle.

As shown in FIGS. 2( a) and 2(b), a significant feature of the despeckle elements 106 is that the staircase element 112 is disposed between the microlens array 114 and the image plane (that is, after the lens array 114 in the direction of travel of the beamlets) and approximately at a focal point (foci) of each of the beamlets 120-1 through 120-3. More specifically, the focal point of beamlet 120-1 is located just before a front surface (i.e., a vertical surface as shown in FIGS. 2( a) and 3) of the corresponding step 112-1, the focal point of beamlet 120-2 is located just at the front surface of the corresponding step 112-2, and the focal point of beamlet 120-3 is located just before or after the front surface of the corresponding step 112-3, according to the thickness of each of the steps 112-1 through 112-3. As a result of this configuration, as shown in FIG. 3, the beamlets 120-1 through 120-3 avoid passing through the horizontal edges 112-4, 112-5, 112-6 and 112-7 forming the horizontal boundaries of the steps 112-1 through 112-3 and which are parallel to the optical path of the beamlets 120-1 through 120-3. Therefore, the beamlets 120-1 through 120-3 are not diffracted at all by the horizontal edges 112-4-112-7, resulting in a significantly improved despeckling. This configuration achieves a remarkably superior despeckling performance of a quality which heretofore has never been achieved in the industry.

In FIGS. 2( a) and 2(b), laser source 102 is illustrated as producing the coherent light beam 116 having a fast axis (FIG. 2( a)) (i.e., larger divergence angle) and a slow axis (FIG. 2( b)) (i.e., smaller divergence angle). The coherent light beam 116 illustrates an elliptically shaped beam. An exemplary beam intensity distribution of coherent light beam 116 is generally a Gaussian distribution, shown in Eq. (1) as:

$\begin{matrix} {{I\left( {x,y} \right)} \propto {\exp\left\lbrack {{- \frac{2x^{2}}{w_{x}^{2}}} - \frac{2y^{2}}{w_{y}^{2}}} \right\rbrack}} & (1) \end{matrix}$

where w_(x), w_(y) are the Gaussian waist size of coherent light beam 116, along the x axis and the y axis, respectively. Although the coherent light beam 116 is illustrated as being elliptically-shaped, it should be understood that the coherent light beam 116 may have any suitable beam shape, including circularly-shaped, symmetrically-shaped, and non-symmetrically-shaped beams.

The laser source 102 may include any suitable laser light source capable of producing coherent light. Examples of laser source 102 include, without being limited to, semiconductor lasers (e.g., laser diodes) including vertical cavity surface emitting lasers (VCSELs), superluminescent diodes (SLDs), light emitting diodes (LEDs), gas lasers, solid-state lasers, disc lasers, and fiber lasers. In general, a coherent light source may be characterized by a coherence length defined by the temporal coherence length times the speed of light in a vacuum, where the coherence length in a material may be scaled by the refractive index of the material. Sources having very narrow bandwidths are typically characterized by higher temporal coherence (and larger coherence lengths) than broadband sources. The example embodiments described below use semiconductor lasers.

Speckle typically occurs due to the relatively long coherence of a laser (i.e., high temporal coherence), to cause a high contrast interference pattern (i.e., a speckle pattern) on the image plane 110. A visibility (i.e., an interference contrast) of the interference pattern due to the coherent light may be represented by Eq. (2) as:

$\begin{matrix} {V = \frac{I_{\max} - I_{\min}}{I_{\max} + I_{\min}}} & (2) \end{matrix}$

where and I_(min) and I_(max) are minimum and maximum intensities, respectively, of the interference pattern. The visibility may be measured, for example, using a Michelson interferometer, as a function of optical path difference between the coherent light split into two light beams. In general, the visibility is typically high in lasers of long coherence, particularly at locations corresponding to small path differences of the split beams.

In order to reduce the visibility of the speckle pattern, the intensity spectrum of the laser source 102 may be broadened. According to an exemplary embodiment, the laser source 102 may be operated at a short pulse (e.g., the gain medium may be driven with a signal having a short pulse, for example, between about 0.5 ns (nanoseconds) to about 100 ns depending on the laser power). By driving the laser source 102 using a short pulse, a multi-mode oscillation occurs in the laser source 102, thus broadening the width of the wavelength band. By broadening the wavelength bandwidth, the visibility degrades, according to the Wiener-Khintchine theorem (owing to the Heisenberg uncertainty principle).

Referring to FIGS. 4( a)-4(d), examples of the wavelength bandwidth broadening and the effect on visibility are shown with respect to an example laser diode operated at approximately 2 ns. In particular, FIG. 4( a) shows an intensity spectrum of the laser diode in continuous wave (CW) operation as a function of wavelength; FIG. 4( b) shows a visibility of the laser diode in CW operation as a function of optical path difference; FIG. 4( c) shows an intensity spectrum of the laser diode in a pulsed operation as a function of wavelength of a laser; and FIG. 4( d) shows a visibility of the pulse-operated laser diode as a function of optical path difference.

As shown in FIG. 4( a), in the CW operation, the laser diode has a very narrow wavelength bandwidth in the intensity spectrum. Because of the narrow wavelength bandwidth, the visibility, as shown in FIG. 4( b), is high across a wide range of optical path differences. The intensity spectrum is related to the visibility through the Fourier transformation according to the Wiener-Khintchine theorem. Qualitatively, a narrower bandwidth in the intensity spectrum produces a higher visibility, i.e., a high coherence. Accordingly, FIG. 4( b) indicates that the laser diode, in CW operation at high frequency, has a high coherence.

In contrast, as shown in FIG. 4( c), in a short pulse operation, the wavelength bandwidth of the laser diode is broadened. Because of the wavelength bandwidth broadening, the visibility, as shown in FIG. 4( d), is substantially reduced across most of the optical path differences. Although a large visibility still exists at main peak 402 (referred to herein as the zero path difference), the remaining peaks 404, which are known as side-mode interference (referred to herein as side-mode peaks), are also substantially reduced (but are not eliminated). The side-mode peaks 404 appear at every optical path difference P calculated by:

p=2nL  (3)

where n and L are the refractive index and the laser cavity length, respectively. Thus, the laser source operated at a sufficiently short pulse has a substantially reduced (i.e., degraded) coherence. Accordingly, by operating laser source 102 (FIGS. 2( a) and 2(b)) at a short pulse, a majority of the interference from different path lengths may be removed.

Referring back to FIGS. 2( a) and 2(b), according to an exemplary embodiment, the laser source 102 may be operated with (driven by) a short pulse of between about 0.5 ns to about 100 ns, and preferably less than 10 nanoseconds. For typical semiconductor lasers, after the injection current is injected to the laser diode chip (i.e., the laser resonator cavity), many modes may be excited for the first few nanoseconds. Particularly during this duration time, the wavelength bandwidth may be significantly broadened, because of the multi-mode operation. After the first few nanoseconds, some modes may quickly decay and, thus, only main modes may remain (based on a rate equation for the carriers and the electrons). Therefore, a pulse width of a few nanoseconds may be a particularly effective duration time to reduce the coherence. However, the coherence also depends on the strength of the injection current. A longer pulse width may still produce a broadened wavelength bandwidth if the injection current is high. Some lasers may have poor coherence even when pulsed at 100 ns, for example. A lower limit of an electronically driven short pulse may be about 0.5 ns.

According to aspects of the present invention, the pulse width may be used to broaden the wavelength bandwidth. The operation frequency, in contrast, may be a function of a desired average output power of the laser source 102. For example, a frequency of 200 MHz may be selected if an average output power of 1 W is desired, for a 2.5 ns pulsed operation with a peak power of 2 W to equivalently achieve a 50% duty ratio. Another example includes a case where only a small output power is desired (for example, as in the application of a laser microscope). In this case, the pulse width is desirably short, while the frequency may be low, which is equivalent to a low duty ratio. Thus, the duty ratio may be determined based on the output power requirement. According to an exemplary embodiment, the laser source 102 may include a single mode laser configured to produce multi-mode oscillation, by being driven with a very short pulse. Multi-mode high power diode lasers may also be used. For display purposes, the laser source 102 may be pulsed with a duty ratio of more than about 50%, to avoid any potentially dangerous high peak power.

The collimator 104 collimates the coherent light beam 116 to form the collimated light beam 118, without substantially changing the beam shape and the beam intensity distribution of the coherent light beam 116. The collimator 104 may be a curved focusing lens having a numerical aperture (NA) defined by n sin θ, where n is the refractive index of the medium and θ is the focusing angle, as shown, or any other collimating configuration of optical elements known to persons skilled in the art. The collimator 104 may have any suitable NA for producing the collimated light beam 118, including, but not limited to, about 0.3.

The NA of the collimator 104 (referred to herein as the collimator NA) may be determined by a desired coupling efficiency, depending on the divergence angles of the laser diodes and an ease of alignment. When the collimator NA is selected so that the focusing angle and the divergence angle of the laser diode are matched, an optimum coupling efficiency may be obtained (i.e., a minimum loss due to vignetting by the collimator aperture is produced). For lasers with very fast divergence angles, a high collimator NA may be selected, for example, greater than or equal to 0.8. The alignment for such a collimator, however, may be difficult, and the collimator may not be tolerable to alignment error. On the other hand, a low collimator NA may be selected, to reduce the alignment constraints and relax the design tolerance. In this case, however, the coupling efficiency is reduced and more light may be lost (because only a portion of the light cone emitted from the laser source 102 may be enclosed inside the collimator 104).

Although the collimator 104 is illustrated as being separate from the laser source 102, it should be understood that the collimator 104 may be integrated with the laser source 102. According to an exemplary embodiment, the collimator 104 may include two separate crossed single axis collimators, each of which may collimate one of the fast or slow axes of a laser diode (as the laser source 102). This configuration may provide an improved coupling efficiency, because the divergence angles of laser diodes may differ significantly between the fast axis and the slow axis.

Referring to FIGS. 2( a), 2(b) and 3, the despeckle elements 106 are further described below. FIG. 3 is a cross-section magnified view of the exemplary despeckle elements 106. As described above, the despeckle elements 106 according to an embodiment include the microlens array 114 and the staircase element 112 disposed after the microlens array 114 and approximately at a focal point (foci) of each of the beamlets 120-1 through 120-3.

The staircase element 112 includes steps 112-1, 112-2, and 112-3 of different heights. The steps 112-1 through 112-3 are configured to form optical path difference generating elements, which remarkably reduce or substantially eliminate speckle. The microlens array 114 includes microlenses 114-1, 114-2, and 114-3, arranged as a one dimensional fly's eye array, and alternatively referred to as a lenticular lens. The microlenses 114-1 through 114-3 are configured to form a fly's eye illumination system and produce a more homogeneously-illuminated field.

As shown in FIG. 3, each step 112-1 through 112-3 is formed in a one-to-one correspondence with each microlens 114-1 through 114-3 (i.e., step 112-1 corresponds to microlens 114-1, step 112-2 corresponds to microlens 114-2, and step 112-3 corresponds to microlens 114-3). In other words, a width and position of each microlens 114-1, 114-2, and 114-3 is in a one-to-one correspondence with a width (W) and position of the respective steps 112-1, 112-2, and 112-3.

Each of the elements of the despeckle element 106, such as the microlens array 114 and the staircase element 112, may be formed of a transparent material having a refractive index (n). Transparent, as used herein, means having substantial optical transmission at those wavelengths at which illumination is intended. The elements of the despeckle element 106 may be formed from any suitable transparent material transparent, such as quartz, BK7, sapphire and other optical grade glass, and transparent plastic materials, such as acrylic and polycarbonate. For example, ZEONEX® (manufactured by ZEON Chemical) is a plastic material suitable for ultraviolet (UV) and UV-blue wavelengths in terms of durability.

In FIGS. 2( a) and 3, three steps (112-1, 112-2, and 112-3) and three microlenses (114-1, 114-2, and 114-3) are shown. It should be understood, however, that the staircase element 112 may have more or less than three steps, and the microlens array 114 may correspondingly have more or less than three microlenses. In general, the staircase element 112 includes N number of steps (where N is greater than 2) (and a corresponding N number of microlenses), so that the steps substantially reduce or eliminate speckle and the microlenses provide a more uniformly illuminated field.

The number of steps and microlenses may be determined according to a desired flat top size and quality of uniformity of the illumination, based on the theory for a fly's eye illumination system, as explained below. Let W and f_(M) stand for the width and focal length of the lenslet of the microlenses, respectively. Let f_(F) be the focal length of the field lens and let n be the refractive index of the medium. For light of wavelength λ, design parameters for the fly's eye illumination system may be given in the following equations.

$\begin{matrix} {{{Flat}\mspace{14mu} {top}\mspace{14mu} {size}\text{:}\mspace{14mu} D} = \frac{{Wf}_{F}}{f_{M}}} & (4) \\ {{{Fresnel}\mspace{14mu} {number}\text{:}\mspace{14mu} F} \approx \frac{W}{f_{M}\lambda}} & (5) \\ {{{Grating}\mspace{14mu} {pattern}\mspace{14mu} {period}\text{:}\mspace{14mu} P} = \frac{f_{F}\lambda}{nW}} & (6) \end{matrix}$

v The flat top size (eq. 4) determines the illumination line length in the one dimensional case and the edge length of the illumination area in the two dimensional case. The Fresnel number (eq. 5) and grating pattern (diffraction) period (eq. 6) determine the quality of the uniformity of the illumination.

In general, the fly's eye illumination system may be designed to produce a sufficiently large Fresnel number, because the uniformity degrades inversely proportional to the Fresnel number. The Fresnel number (eq. 5) represents how many diffraction rings exist in a Fresnel diffraction pattern. In the fly's eye illumination system, each beamlet passing through each lenslet in the microlenses produces a Fresnel diffraction pattern. Each of the Fresnel diffraction patterns produced at each lenslet are superimposed on the image plane and are averaged to form a uniform illumination. If the number of Fresnel diffraction rings is small, large waves exist in a Fresnel diffraction pattern, which may not be averaged or eliminated by the superposition. Thus, a small Fresnel number may produce a poor illumination uniformity.

On the other hand, larger Fresnel numbers produce more waves, i.e., many smaller waves in a Fresnel diffraction pattern. The many and smaller waves are washed out and become substantially invisible in the averaged image. The diffraction period (eq. 6) is another indicator of the roughness in the illumination. Diffraction may occur in the fly's eye illumination system, because the fly's eye lens may act as a grating (due to the periodic edges of each lenslet). This diffraction appears on the image plane as a periodic diffraction pattern with the minimum period given by Eq. (6).

Each step 112-1 through 112-3 has a width W and a height H, with a total thickness T. In FIG. 3, the steps 112-1 through 112-3 are arranged in a staircase configuration. The width W may be determined by a desired system specification but, more importantly, may be determined by considering, as determined by the present inventors, that the staircase diffracts the incident light beam at the horizontal edge of the staircase, as described above. This diffraction perturbs the function of the fly's eye illumination system, by splitting the incident beam into at least 0th order diffracted light and +/−1st order diffracted light. The separation of the incident beam is approximately calculated by

$\begin{matrix} {\frac{T\; \lambda}{nW},} & (7) \end{matrix}$

where n is the refractive index of the staircase element 112. In some instances, it may be desirable to select a larger W, to minimize this separation in order to obtain a more uniform illumination on the image plane. It may also be desirable to select a larger beam size, to include a sufficient number of lenslets for a more pronounced averaging effect.

Furthermore, as shown in FIG. 3, the staircase element 112 and microlens array 114 are separated by a distance D, which is calculated to approximately correspond to the focal distance of the beamlets 120, so that the beamlets 120 focus right at or near the center of the vertical edges of the steps 112-1 through 112-3. By choosing an appropriate distance D, the despeckling elements 106 according to aspects of the present invention prevent the beamlets 120 from being diffracted by the horizontal edges 112-4 through 112-7, thereby achieving a superior despeckling ability.

As shown in FIG. 3, the steps 112-1 through 112-3 monotonically increase in height (i.e., are arranged in a staircase configuration), so that each step has a different height. The steps 112-1 through 112-3 may also be arranged to have randomly varying heights. The optical path difference (OPD) produced by a step with height H is given by

(n−1)H  (8)

In FIG. 3, there is an OPD of (n−1)H between step 112-1 and 112-2; 2(n−1)H between step 112-1 and 112-3; and (n−1)H between step 112-2 and 112-3. According to an exemplary embodiment, there may be a path difference between any combination of arbitrary steps among all of the steps. Thus, any combination of two arbitrary beamlets 120 among all of the beamlets 120 has a path difference and thus has little or no correlation. Hence, speckle may be substantially reduced or eliminated. In FIG. 3, steps 112-1 to 112-3 monotonically increase by H as 0, H, and 2H but may be configured with any different heights. For example, as 0, H, 3H; or 1H, 3H, 5H. Also steps 112-1 to 112-3 may randomly increase. For example, as 0, 2H, 1H; 0, 3H, H; or 1H, 5H, 3H. In an exemplary embodiment, steps 112-1 to 112-3 each have a height H of less than about 1 mm.

As described above, the laser source 102 is configured to provide the coherent light beam 116 having a substantially reduced coherence. However, as shown in FIG. 4( d), there is still a high coherence (i.e., interference) at the zero path difference peak 402. Accordingly, the steps 112-1 to 112-3 may be configured as optical path difference elements, to substantially reduce or eliminate the remaining interference (i.e., interference not reduced by the laser source 102).

If an optical path difference is introduced between two (or more) portions of the coherent light beam 116 that exceeds the coherence length, the ability for interference to occur between the portions is substantially reduced. Accordingly, all of the beamlets 120 emerging from a surface of the microlens array 114 become interference free (i.e., having substantially no speckle). Because the pulsed laser source 102 substantially reduces the coherence except for the zero path difference peak 402, the height H of the steps 112-1 to 112-3 may be selected to be greater than the coherence length and less than the first coherence revival length (i.e., the length to the first side-mode peak 404 (FIG. 4( d)) from the zero path difference peak 402). For example, referring to FIG. 4( d), if the zero path difference peak 402 drops to nearly zero at a path difference of about 0.5 mm, the optical path difference element may be configured to have a step height of 0.5/(n−1) (taking into account the refractive index n of the material of the despeckle element 106). It should be understood that a step height of 0.5/(n−1) merely represents one example.

For example, the step height H may be selected as 1 mm for the staircase element 112 having a refractive index of 1.5, because the minimum OPD is (n−1)H=0.5 mm. For a step height of 0, H, 2H for the three steps 112-1 to 112-3, the OPDs are 0.5 mm, 1.0 mm and 1.5 mm, respectively and the visibility for all of the OPDs is nearly zero. For more than the three steps 112-1 to 112-3, one or more OPDs of all possible OPDs may match the length of the side-mode peak 404 (FIG. 4( d)). It may be desirable to design the beam size, microlens size, step size, and step height so that the any of the OPDs are far enough from the side-mode peaks 404 (FIG. 4( d)). The OPD may be even larger than the first side-mode peak 404 (FIG. 4( d)) if there are no limitations in the physical size.

Although the steps 112-1 to 112-3 are illustrated as having a same width W, the width of each step may be individually varied. Furthermore, although the steps 112-1 to 112-3 are illustrated as each having a monotonically increasing height, it is understood that the height H of each step may also be individually varied. It is further understood that a radius of curvature for individual microlenses 114-1 to 114-3 may be adjusted to compensate for any variation in the width W of the steps 112-1 to 112-3, so that the microlenses all have the same focal length.

As illustrated in FIG. 3, the staircase element 112 includes physical steps 112-1 to 112-3 arranged as a staircase to introduce optical path differences, in order to substantially remove any coherence from the collimated light beam 118. However, the staircase element 112 is not limited to physical steps to reduce the coherence. In general, the steps 112-1 to 112-3 represent optical steps that may be used to reduce the coherence. The staircase element 112 may also include differences in polarization (described below with respect to FIGS. 6 and 7) and differences of refractive index. For example, different refractive indices may be introduced into the staircase element 112 of despeckle element 106 by selection of material or by coating, or doping, or implantation of materials, or in any other manner known to those skilled in the art.

Accordingly, the despeckle elements 106 provide a reduction in coherence, based on the staircase element 112. In addition, the despeckle elements 106 include the microlens array 114, which splits the collimated light beam 118 into a plurality of beamlets 120-1, 120-2, 120-3, such that the number of beamlets 120 (e.g., three beamlets 120-1, 120-2, and 120-3) correspond to the number of microlenses (e.g., three microlenses 114-1, 114-2, and 114-3). The microlenses 114-1, 114-2, and 114-3 are configured to focus the beamlets 120 to a point before or onto field lens 108.

If the despeckle elements 106 only included the microlens array 114, without the staircase element 112, the microlenses 114-1, 114-2, and 114-3 would produce a more homogenously illuminated field at the image plane 110. However, the beamlets 120 would still interfere with each other and produce speckle.

Interference (i.e., speckle) may occur when multiple beamlets 120 come together at one spatial point. In conventional illumination systems using coherent light sources, interference may be caused by microlenses as they split a collimated light beam into multiple beamlets. Accordingly, it is desirable to ensure that the beamlets 120 from each of the microlenses 114-1, 114-2, and 114-3 have a reduced correlation, to avoid interference at the image plane 110. According to aspects of the present invention, by providing a one-to-one correspondence between the steps 112-1 to 112-3 and the microlenses 114-1 to 114-3, interference between the beamlets 120 is reduced.

In FIGS. 2( a), 2(b) and 3, the despeckle elements 106 are illustrated as a one dimensional array, with a one dimensional array of steps 112-1 to 112-3 and a one dimensional array of microlenses 114-1 to 114-3 extending in the fast axis. In this example, the microlenses 114-1 to 114-3 may be formed as lenticular lenses. It is to be understood, however, that the despeckle element 106 is not limited to a one-dimensional array and may include a one-dimensional crossed configuration (FIGS. 11( a), 11(b), 11(c), 12, 13 and 14) or a two-dimensional array configuration (FIGS. 15, 16(a) and 16(b)). For example, if the coherent light beam 116 has a circular beam shape, the despeckle element 106 may be formed as a one-dimensional crossed array of steps and microlenses, described further below with respect to FIGS. 11( a), 11(b), 11(c), 12, 13 and 14.

Referring back to FIGS. 2( a) and 2(b), the beamlets 120 are directed to the field lens 108. The field lens 108 (e.g., a Fourier lens) superimposes the multiple beamlets 120 together at the image plane 110 (e.g., a specimen position) located near a focus position, leading to a homogenously illuminated field. The field lens 108 may be positioned anywhere between the despeckle elements 106 and the image plane 110. The position of the field lens 108 may be used to change the energy distribution (e.g., from a Gaussian profile to a flat-top profile) at the image plane 110 by coarse positioning across the focus or to change the energy level of a flat-top profile by fine positioning across near focus.

According to aspects of the present invention, the exemplary homogenizer 100 produces the coherent light beam 116 with reduced coherence and includes despeckle elements 106, which further reduce the coherence. Accordingly, homogenizer 100 effectively eliminates speckle, with a static configuration of elements, where the size of the elements may be very small. By including the microlens array 114 and the staircase element 112 disposed after the microlens array 114 at approximately the focal points of the beamlets 120, the averaging effect by the beamlets 120 is increased.

FIG. 5 illustrates a laser beam homogenizer 500 according to a second embodiment. The laser beam homogenizer 500 according to the second embodiment includes various elements which are identical to the elements included in the laser beam homogenizer 100 according to the first embodiment, and a detailed description of these elements is omitted.

As shown in FIG. 5, the laser beam homogenizer 500 includes despeckle elements 506 which include the microlens array 114 and the staircase element 112 of the first embodiment, along with a second microlens array 504 (fly's eye lens array) disposed after the staircase element 112 which receives the beamlets 120 transmitted from the staircase element 112. By disposing a second microlens array 504 after the staircase element 112 to receive the beamlets 120, the laser beam despeckle elements 506 achieve a tandem configuration which achieves improved uniformity in the flat top profile generated by the laser beam homogenizer 500. It is to be understood by those skilled in the art that the second microlens array 504 can be disposed in various positions (e.g, closer to or farther away from the staircase element 112) according to various criteria known to those skilled in the art.

FIG. 6 illustrates a despeckle element 600 according to a third embodiment. As shown in FIG. 6, the despeckle elements 600 according to a third embodiment include a staircase element 612 including a series of steps 612-1, 612-2, and 612-3, with each step having an optical waveplate 602 disposed thereon. In particular, the left side of FIG. 6 illustrates a cross-sectional diagram of despeckle elements 600, and the right side of FIG. 6 illustrates a cross-section diagram of the polarization directions for polarized light 612-P and 612-S passed from a surface of the staircase element 612 of the despeckle elements 600. Despeckle element 600 is similar to despeckle element 106 (FIG. 1), except that despeckle element 600 includes optical wave plates 602. FIG. 6 illustrates three steps 612-1, 612-2, and 612-3 and three optical wave plates 602-1, 602-2, and 602-3 respectively disposed thereon, but it is understood that more or less than three steps and three waveplates can be used.

Despeckle element 600 includes a staircase element 612 formed of a transparent material and having physical steps 612-1, 612-2 and 612-3 formed in a staircase configuration as optical path difference elements, as described above. In addition, despeckle element 600 includes a respective wave plate 602 on a portion of each physical step 612. Wave plate 602 is used to alter the polarization state of incident light 610 received by despeckle element 600. The staircase element 612 and the waveplates 602 disposed on the staircase element 612 are positioned around the foci of a microlens array (not shown) to avoid undesired diffraction.

In despeckle element 600, each wave plate 602 also represents an optical step. Accordingly, microlenses of a microlens array (not shown) used in conjunction with the despeckle element 600 are in a one-to-one correspondence with optical steps (physical steps 612-1, 612-2 and 612-3 and wave plates 602-1, 602-2 and 602-3) of despeckle element 600. Thus, the despeckle element 600 is configured to be used with a microlens array having six microlenses respectively corresponding to physical step 612-1, optical waveplate 602-1, physical step 612-2, optical waveplate 602-2, physical step 612-3, and optical waveplate 602-3. For example, the microlens array 114 of FIG. 1 could be modified to include six microlenses instead of the three microlenses 114-1, 114-2, and 114-3 shown in FIG. 1, to be compatible with the despeckle element 600.

In an exemplary embodiment, wave plate 602 includes a half wave plate, which changes the polarization direction of linear polarized light (i.e., by rotating polarization axis A by 90°, making it orthogonal to incident beam 610). The despeckle element 600 may be used, for example, instead of the despeckle element 106 (as shown in any of FIG. 2( a), 2(b), 3, or 4) or despeckle element 506 (FIG. 5), with the addition of a polarizer (not shown) in the optical path between collimator 104 and field lens 108.

As shown in FIG. 6, in operation, incident light beam 610 having polarized light (for example, p polarized light with a polarization direction indicated by arrow A), passes through despeckle element 600 to produce p-polarized light 612-P and s-polarized light 612-S.

Polarized light 612-P (passed through physical steps 612-1, 612-2, and 612-3, but not passed through optical waveplates 602-1, 602-2, and 602-3) are passed without any change in the polarization direction (i.e., as p-polarized light). Furthermore, incident light beam 610 is also subject to optical path differences, due to the difference in step heights of physical steps 612-1, 612-2 and 612-3. Because of the optical path differences of steps 612-1, 612-2 and 612-3, polarized light 612-P passing through, for example, physical step 612-1 does not interfere with polarized light 612-P passing through, for example, physical step 612-2 and, similarly, polarized light 612-P passing through, for example, physical step 612-2 does not interfere with polarized light 612-P passing through, for example, physical step 612-3.

Polarized light 612-S (passed through optical waveplates 602-1, 602-2, and 602-3, respectively) is passed with a change in the polarization direction. In addition, incident light beam 610 is subject to optical path differences, due to the differences in step heights of physical steps 612-1, 612-2 and 612-3. Because of the optical path difference of steps 612-1, 612-2 and 612-3, polarized light 612-S passing through, for example, optical waveplate 602-1 does not interfere with polarized light 612-S passing through, for example, optical waveplate 602-2, and, similarly, polarized light 612-S passing through, for example, optical waveplate 602-2, does not interfere with polarized light 612-S passing through, for example, optical waveplate 602-1.

Since linearly polarized (e.g., p-polarized) and orthogonally polarized (e.g., s-polarized) beams do not interfere with each other, no step needs to be added to one of the two adjacent positions on the staircase configuration. Accordingly, wave plates 602-1, 602-2 and 602-3 may be formed directly on the physical steps 612-1, 612-2, and 612-3 without increasing the step height. Accordingly, a thickness of the despeckle element 600 may be reduced to half of the thickness and half the number of physical steps of a despeckle element where the optical steps are formed only using physicals steps as optical path difference elements (e.g., three physical steps in FIG. 6 as opposed to six physical steps of a corresponding despeckle element similar to despeckle element 106 of FIG. 2( a) but having six steps). Thus, the despeckling element 600 according to a third embodiment of the present invention has a reduced total thickness and is very compact.

Furthermore, although FIG. 6 illustrates the wave plates 602 as being disposed on the steps 612-1, 612-2 and 612-3 of the staircase element 612, it is understood that the wave plates 602 are not limited to being disposed on the staircase element 612, and may instead be disposed on another element. In this case, the waveplates 602 should still preferably be positioned around the foci of the microlens array to avoid undesired diffraction. Moreover, although FIG. 6 illustrates a respective wave plate 602 on a portion of each physical step 612-1, 612-2 and 612-3, wave plates 602 may also be placed on every other physical step, or in other arrangements known to those skilled in the art.

FIG. 7 illustrates a despeckle element 700 according to a fourth embodiment. As shown in FIG. 7, the despeckle element 700 according to a fourth embodiment includes first and second optical wave plates 702 and 704. In particular, the left side of FIG. 7 is a cross-section diagram of despeckle element 700, and the right side of FIG. 7 is a cross-section diagram illustrating polarization directions for polarized light 712-L and 712-R passed from the staircase element 712 of the despeckle element 700. Despeckle element 700 is similar to despeckle element 600 (FIG. 6), except that despeckle element 700 includes respective first and second optical wave plates 702 and 704.

The despeckle element 700 includes a staircase element 712 formed of a transparent material and including physical steps 712-1, 712-2 and 712-3. The physical steps 712-1, 712-2 and 712-3 are formed in a staircase configuration as optical path difference elements, as described above. In addition, the despeckle element 700 includes first and second wave plates 702 and 704 on each physical step 712-1, 712-2 and 712-3. First and second wave plates 702 and 704 may be used to alter the polarization state of incident light beam 710 received by the despeckle element 700. The staircase element 712 and the waveplates 702 and 704 disposed on the staircase element 712 are positioned around the foci of a microlens array (not shown) to avoid undesired diffraction.

In the despeckle element 700, first and second wave plates 702 and 704 also represent optical steps. Accordingly, microlenses of the microlens array (not shown) are in a one-to-one correspondence with the optical steps (first and second wave plates 702 and 704). Thus, the despeckle element 700 is configured to be used with a microlens array having six microlenses respectively corresponding to first waveplate 702-1, second waveplate 704-1, first waveplate 702-2, second waveplate 704-2, first waveplate 702-3, and second waveplate 704-3. For example, the microlens array 114 of FIG. 1 could be modified to include six microlenses instead of the three microlenses 114-1, 114-2, and 114-3 shown in FIG. 1, to be compatible with the despeckle element 700.

In an exemplary embodiment, first wave plate 702 includes a quarter wave plate and second wave plate 704 includes a three-quarter wave plate. The quarter wave plate (i.e., first wave plate 702) changes linearly polarized light to right circular polarized light and the three-quarter wave plate (i.e., second wave plate 704) changes linearly polarized light to left circular polarized light. The despeckle element 700 may be used, for example, instead of despeckle element 106 (as shown in any of FIG. 2A, 2B, or 3, 4, 7A-8D) or despeckle element 506 (FIG. 5), with the addition of a polarizer (not shown) in the optical path between collimator 104 and field lens 108.

As shown in FIG. 7, in operation, the incident light beam 710 having linearly polarized light (for example, p polarized light with a polarization direction indicated by arrow A), passes through despeckle element 700 to produce right-circular-polarized light 712-R and left-circular-polarized light 712-L.

Polarized light beams 712-R have right circular polarization (from respective first wave plates 702-1, 702-2, and 702-3). In addition, incident light beam 710 is subject to optical path differences, due to the difference in step heights of physical steps 712-1, 712-2, and 712-3. Because of the optical path difference of steps 712-1, 712-2, and 712-3, polarized light 712-R passing through, for example, first wave plate 702-1 does not interfere with polarized light 712-R passing through, for example, first wave plate 702-2 and, similarly, polarized light 712-R passing through, for example, first wave plate 702-2 does not interfere with polarized light 712-R passing through, for example, first wave plate 702-3.

Polarized light beams 712-L have left circular polarization (from respective second wave plates 704-1, 704-2 and 704-3). In addition, incident light beam 710 is subject to optical path differences, due to the difference in step heights of physical steps 712-1, 712-2, and 712-3. Because of the optical path differences of steps 712-1, 712-2, and 712-3, polarized light 712-L passing through, for example, second wave plate 704-1 does not interfere with polarized light 712-L passing through, for example, second wave plate 704-2 and, similarly, polarized light 712-L passing through, for example, second wave plate 704-2 does not interfere with polarized light 712-L passing through, for example, second wave plate 704-3.

Since right circular polarization and left circular polarization beams do not interfere with each other, no step needs to be added to one of the two adjacent positions on the staircase configuration. Accordingly, both first wave plate 702 and second wave plate 704 may be formed directly on staircase element 712 without increasing the step height. Thus, the thickness of despeckle element 700 may be reduced to half of the thickness and half the number of physical steps (e.g., three physical steps as opposed to six physical steps) compared to a step of despeckle element 106 (FIG. 2( a)) but having six steps. Thus, the despeckling element 700 according to a fourth embodiment of the present invention has a reduced total thickness and is very compact.

Furthermore, although FIG. 7 illustrates the wave plates 702 and 704 as being disposed on the steps 712-1, 712-2, and 712-3 of the staircase element 712, it should be understood that the wave plates 702 and 704 are not limited to being disposed on the staircase element 712, and may instead be disposed on another element. In this case, the waveplates 702 and 704 should still preferably be positioned around the foci of the microlens array to avoid undesired diffraction. Moreover, although FIG. 7 illustrates a wave plate 702 and a wave plate 704 on each physical step 712-1, 712-2, and 712-3, wave plates 702 and 704 may also be placed on every other physical step, or in other arrangements that achieve similar effects to those described above.

FIGS. 8( a) and 8(b) illustrate an exemplary despeckling laser array 800 (also referred to herein as “array 800”) according to a fifth embodiment of the present invention. In particular, FIG. 8( a) illustrates a cross-sectional diagram of array 800 with respect to a slow axis of laser sources 102, and FIG. 8( b) illustrates a cross-section diagram of array 800 with respect to a fast axis of laser sources 102.

The array 800 is similar to the laser beam homogenizer 100 (FIGS. 2( a) and 2(b)), except that the array 800 includes a plurality of laser sources 102-1, 102-2, and 102-3 having a plurality of corresponding collimators 104-1, 104-2, 104-3 and a plurality of corresponding despeckle elements 806-1, 806-2 and 806-3. Beamlets from the plurality of despeckle elements 806-1, 806-2 and 806-3 are superimposed together by field lens 808 at the image plane 810.

Because laser sources 102-1, 102-2, 102-3 are independent laser sources, they are not correlated with each other and do not coherently interfere with each other. Thus, beamlets from the plurality of despeckle elements 806-1, 806-2, and 806-3 may be combined by a common field lens 808 and may overlap at the image plane 810. The combined beam profile is thus averaged out and may produce a more uniform intensity profile.

As shown in FIGS. 8( a) and 8(b), the array 800 is configured to have three despeckle elements 806-1, 806-2 and 806-3 arranged in a single row and 3 columns (one column for each of the despeckle elements) (a 3×1 configuration). However, the array 800 is not limited to being configured in this fashion, and instead may be configured in any arbitrary way, such as, for example, a 3×2 configuration, a 6×3 configuration, etc. Also, when the array 800 is configured to include 1-dimensional despeckling elements, such as despeckling elements 806-1, 806-2, and 806-3, the array 800 can be used for various purposes, such as laser annealing. In this case, it is preferable that the beam shaping (flat top making) axis be located on the side of the array having the smaller number of either the rows or columns employed in the array, for example, the 1 row side (FIG. 8( b)) for a 3×1 array, the 2 row side for a 3×2 array, and the 3 row side for a 6×3 array, and that the axis with the larger number of the rows or columns be chosen as the non-shaping axis. This arrangement facilitates the design of the field lens. In the beam shaping axis, there are microlens arrays (fly's eye lens arrays) which diverge or converge beamlets, thereby making certain field angles against the field lens. When the field lens needs to be designed for the wider field and wider field angle (i.e., the larger of the rows or columns), the design of the field lens becomes more difficult. Therefore, the axis of the shorter length (i.e., the fewer of the rows or columns in the array), such as, for example, the short axis shown in FIG. 8( b), is preferable to be used for beam shaping. On the other hand, the other axis, for example, the long axis in FIG. 8( a), has no microlens array and only transmits a collimated beam. Accordingly, this makes the design of the field lens simpler and easier.

The despeckle elements 806-1, 806-2 and 806-3 may be the same as the despeckle elements according to other embodiments, for example, the despeckle element 106 according to a first embodiment (FIG. 1). Furthermore, the array 800 may also include two or more despeckle elements 806 per laser source 102, instead of only one. Also, the array 800 may also include an additional microlens array, as described above with respect to FIG. 5. The despeckle elements 806 may also include one or more optical wave plates, as described above with respect to FIGS. 6 and 7. It should be understood that any one or more of the embodiments described herein may be combined into one optical system including a common field lens 808.

FIGS. 9( a) and 9(b) illustrate an exemplary despeckling laser array assembly 900 (also referred to herein as “assembly 900”) according to a sixth embodiment of the present invention. In particular, FIG. 9( a) is a side-plan view diagram of assembly 900 with respect to the x and y axes; FIG. 9( b) is a cross-section diagram along line 9B of assembly 900 with respect to the x and z axes (relative to non-shaping axis); and FIG. 9( c) is a cross-section diagram along line 9C of assembly 900 with respect to the y and z axes (relative to beam shaping axis).

Assembly 900 includes a plurality of laser sources 902 each having a corresponding collimator 904 and despeckle element 906. Each of the despeckle elements 906 includes a microlens array 914 and a staircase element 912, and may have the same configuration as the despeckle elements according to other embodiments of the present invention. Beamlets from the plurality of microlens arrays 914 are combined by a common field lens 908. The assembly 900 also includes a plurality of driver integrated circuits (ICs) 901 mounted on a printed circuit board 903. The driver ICs 901 may be configured to drive respective laser sources 902.

The laser source 902, collimator 904, despeckling element 906 (including the microlens array 914 and the staircase element 912) and the field lens 908 form an assembly 900. Assembly 900 is similar to array 800 (FIGS. 8( a) and 8(b)), except that assembly 900 is assembled in chassis 918. Although in an exemplary embodiment chassis 918 is formed from molded aluminum, chassis 918 may be formed from any material suitable for housing array 922.

Each laser source 902 may be mounted in a separate holder. Each holder may be adhered to chassis 918 after optical axis adjustment with respect to tilt and/or x/y correction (i.e., the correction of tilts of the incident light beam with respect to optical axis 922). Each laser source 902 may then be electrically connected to a respective driver IC 901 via circuit board 903. Each collimator 904 may be mounted in respective holder 926, where holder 926 may include adjustment notch 928 (shown in FIG. 9( c)). Notch 928 may be configured to move holder 926 along optical axes 922, 924, in order to adjust the amount of collimation.

The despeckle elements 906 and field lens 908 may be adhered to chassis 918. Once the array 922 is suitably secured in the chassis 918, top lid 930 may be placed on chassis 918 and may be secured to chassis 918 (for example, via screws). Chassis 918 may be secured to bottom base plate 920 (for example, via bolts).

Although the despeckle elements 906 are positioned as shown in FIG. 9( b), the despeckle elements 906 may be positioned according to any of the configurations described above with respect to any of the other embodiments. Furthermore, although assembly 900 is illustrated as including despeckle elements 906, it is understood that assembly 900 is not limited to the illustrated configuration. Furthermore, although FIGS. 9( a)-9(c) illustrate laser sources 901 having elliptical beam shapes, laser sources 901 may have a circular beam shape. Accordingly, despeckle elements 906 may be replaced with one-dimensional crossed despeckle elements (described below with reference to FIGS. 11( a)-13). Despeckle elements 906 may also include one or more optical wave plates, as described above with respect to FIGS. 6 and 7. It should be understood that any one or more of the embodiments described herein may be combined into assembly 900.

FIG. 10 illustrates a top-plan view diagram of an exemplary system 1000 according to a seventh embodiment of the present invention. As shown in FIG. 10, the exemplary system 1000 is used for annealing a substrate 1002. System 1000 includes a two-dimensional arrangement of assemblies 900 (FIGS. 9( a)-9(c)) along column and row directions, configured to produce annealing lines 1004 (i.e., annealed portions) of substrate 1002. Substrate 1002 may include any suitable substrate for being annealed by laser sources. For example, substrate 1002 may include, without being limited to, amorphous silicon for large organic LED displays.

Assemblies 900 may be shifted or displaced relative to each other in the column direction by an amount Xa. A beam line width Lb of annealing lines 1004 is typically smaller than a width La of assembly 900. Accordingly, in order to anneal the entire surface of substrate 1002, assemblies 900 may be arranged in an interlace configuration (e.g., similar to an inkjet line head), such that assemblies 900 are shifted by an amount Ya in the row direction.

For a beam line width Lb which is equal to an annealing width, a gap Sb between annealing lines 1004 may be selected with respect to laser array width La and a gap Sa between assemblies 900 according to Eq. (9) as:

(L _(b) −S _(b))M=L _(a) +S _(a)  (9)

for a total number of columns M for the case of one scanning period. The shift Ya of assemblies 900 in the row direction may be given by Eq. (10) as:

Y _(a) =L _(b) +S _(b)  (10)

The shift Xa of assemblies 900 in the column direction may be arbitrarily selected. For a given shift Xa, a total length of system 1000 in the column direction becomes MXa.

In FIG. 10, a total of M×N number of assemblies 900 are positioned above substrate 1002, where N represents a total number of rows. FIG. 10 represents an example embodiment of system 1000. It is understood that system 1000 may include fewer columns of assemblies 900, to scan the entire surface of substrate 1002. For example, fewer columns of assemblies 900 may be scanned multiple times while being shifted in the row direction (similar to operation of a serial inkjet printer).

FIG. 11( a) depicts a one-dimensional crossed staircase element 1100 according to an eighth embodiment of the present invention. As shown in FIG. 11, the one-dimensional crossed staircase element 1100 includes a first staircase element 1102 (also referred to herein as “horizontal staircase element 1102”) and a second staircase element 1104 (also referred to herein as “vertical staircase element 1104”). The one-dimensional crossed staircase element 1100 according to aspects of the present invention achieves outstanding despeckling results for various applications which require relatively high amounts of power, such as laser annealing and laser displays.

The horizontal staircase element 1102 includes a series of three horizontal steps 1102-0, 1102-1 and 1102-2. The horizontal staircase element 1102 may be formed of any transparent material known to those skilled in the art, as described above with respect to the other embodiments. According to an aspect of the present invention, the first step 1102-0 has a step height of 0, and each of the steps 1102-1 and 1102-2 has a step height of approximately 3 mm. However, it is understood that the steps 1102-1 and 1102-2 are not limited to this height, but that any of the steps may be various other heights depending on design conditions. It is further understood that, although in this embodiment, a step height of “0” refers to step 1102-0 literally having an absence of any height (i.e., an absence of any material in the step), it is understood that a step having a “step height of 0” is not limited to a step having a complete absence of height/material, and can alternatively refer to a reference step height wherein the step is formed with some small amount of material and therefore has a non-zero height which is the reference height “0” relative to the other steps. That is, a step height of “0” may refer to a relative height of the step in comparison to other steps, as would be appreciated by those skilled in the art.

The vertical staircase element 1104 includes a series of three vertical steps 1104-0, 1104-1 and 1104-2. The vertical staircase element 1104 may be formed of the same materials as the horizontal staircase element 1102, or may be formed of different materials. According to an aspect of the present invention, the first step 1104-0 has a step height of 0 (which, as explained above with respect to the horizontal staircase element 1102, may refer to either a literal absence of height or may be a zero reference height), and each of the steps 1104-1 and 1104-2 has a step height of approximately 1 mm. However, it is understood that the steps 1104-1 and 1104-2 are not limited to this height, but that any of the steps may be various other heights depending on design conditions. In the illustrated embodiment, the step heights of the steps 1102-1 and 1102-2 of the horizontal staircase element 1102 are selected to be approximately three times the step heights of the steps 1104-1 and 1104-2 of the vertical staircase element 1104, for reasons explained further below.

The one-dimensional crossed staircase element 1100 may be made of the same transparent material described above with respect to the other embodiments, and may be formed in various ways. For example, the horizontal staircase element 1102 and vertical staircase element 1104 may be separately formed and then adhered together, using any adhesive material known to those skilled in the art.

FIGS. 11( b) and 11(e) illustrate the one-dimensional crossed staircase element 1100 shown in FIG. 11( a) in use, viewed from a top perspective and a side perspective respectively. In particular, FIG. 11( b) illustrates the one-dimensional crossed staircase element 1100 viewed from a top perspective along a fast axis, and FIG. 11( c) illustrates the one-dimensional crossed staircase element 1100 viewed from a side perspective along a slow axis.

As shown in FIG. 11( b), laser light 1110 passes through a one-dimensional crossed microlens array 1114 (a fly's eye lens array) and is transmitted to the one-dimensional crossed staircase element 1100. Collectively, the “one-dimensional crossed microlens array 1114” and the “one-dimensional crossed staircase element 1100” may also be referred to as a “one-dimensional crossed despeckling unit.” The laser light 1110 can be generated by, for example, a laser diode as described with respect to other embodiments of the invention. The one-dimensional crossed microlens array 1114 includes two microlens arrays 1116, including a first microlens array 1116-1 (FIG. 11( b)) which is configured to focus the laser light 1110 along the fast axis, and a second microlens array 1116-2 (HG. 11(b)) disposed on an opposite side of the first microlens array 1116-1 and which is configured to focus the laser light 1110 along the slow axis.

The number of microlenses 1117 in the first microlens array 1116-1 is set to match the number of steps in the horizontal staircase element 1102. To achieve maximum despeckling results, it is essential that the number of microlenses 1117 match the number of steps in the horizontal staircase element 1102. In this particular embodiment, the first microlens array 1116-1 includes three microlenses 1117-1, 1117-2, and 1117-3, and the horizontal staircase element 1102 includes three corresponding steps 1102-0, 1102-1, and 1102-2. Although the step 1102-0 has a step height of zero, this step is still considered a step, and thus has a corresponding microlens (1117-3). Furthermore, the reason why there are only two steps with a non-zero step height (i.e., steps 1102-1 and 1102-2) in the horizontal staircase element 1102 is to make the total thickness of the horizontal staircase element 1102 as thin as possible for compactness, by making the first step height of step 1102-0 equal to zero. Thus, since the horizontal staircase element 1102 has three steps 1102-0, 1102-1 and 1102-2, the first microlens any 1116-1 has three corresponding microlenses 1117-1, 1117-2 and 1117-3. In this way, along the fast axis, a first one of the microlenses can transmit light through both steps with a non-zero step height and the step with zero step height (i.e., steps 1102-2, 1102-1, and 1102-0) to achieve a maximum step height, a second one of the microlenses can transmit light through one of the steps with a non-zero step height and the step with zero step height (i.e, steps 1102-1 and 1102-0) to achieve an intermediate step height, and a third one of the microlenses can transmit light through only the step with a zero step height (i.e., step 1102-0) to achieve a minimum step height.

Similarly, the number of microlenses 1118 in the second microlens array 1116-2 is set to match the number of steps in the vertical staircase element 1.104. Thus, since the vertical staircase element 1104 has three steps 1104-0, 1104-1 and 1104-2, the second microlens array 1116-2 has three corresponding microlenses 1118-1, 1118-2 and 1118-3. The step 1104-0 is configured to have a step height of 0, for the same reasons as mentioned above with respect to the step 1102-0 in the horizontal staircase element 1102 (i.e., to achieve a very compact design), and the steps 1104-1 and 1104-2 are configured to have non-zero step heights. In this way, along the slow axis, a first one of the microlenses can transmit light through both steps with a non-zero step height and the step with zero step height (i.e., steps 1104-2, 1104-1 and 1104-0) to achieve a maximum step height, a second one of the microlenses can transmit light through one of the steps with a non-zero step height and the step with zero step height (i.e., steps 1104-1 and 1104-0) to achieve an intermediate step height, and a third one of the microlenses can transmit light through only the step with zero step height (i.e., step 1104-0) to achieve a minimum step height.

The one-dimensional crossed microlens array 1114 can be formed in various ways. For example, the first microlens array 1116-1 and the second microlens array 1116-2 can each be the same as the microlens arrays described with respect to other embodiments of the present invention (e.g., microlens array 114 described in embodiment 1). The first microlens array 1116-1 and the second microlens array 1116-2 can be integrally formed from the same transparent material, or can be separately formed.

As shown in FIG. 11( b), in the top view plane, when the laser light 1110 is passed through the one-dimensional crossed microlens array 1114, the microlenses 1117-1, 1117-2 and 1117-3 focus the laser light 1110 into three column of beamlets. Similarly, in the side view plane, when the laser light 1110 is passed through the one-dimensional crossed microlens array 1114, the microlenses 1118-1, 1118-2 and 1118-3 focus the laser light 1110 into three row of beamlets resulting in 3×3 nine beamlets 1110-1, 1110-2, 1110-3, 1110-4, 1110-5, 1110-6, 1110-7, 1110-8 and 1110-9, which are then transmitted to the one-dimensional crossed staircase element 1100. The one-dimensional crossed staircase element 1100 is disposed near the foci of the laser light 1110 passed through the one-dimensional crossed microlens array 1114, to prevent edges 1108 of both the horizontal staircase element 1102 and vertical staircase element 1104 which are parallel to the optical path of the beamlets from creating unnecessary diffraction of the beamlets 1110-1, 1110-2, and 1110-3, and thereby achieving superior despeckling performance.

A column of beamlets 1110-7, 1110-8 and 1110-9 passes through steps 1102-2, 1102-1 and 1102-0 of the horizontal staircase element 1102. In this illustration, the step heights of the steps 1102-1 and 1102-2 are each 3 mm, and the step heights of the steps 1104-1 and 1104-2 are each 1 mm, and the step heights of 1102-0 and 1104-0 are zero, as an example. As shown in FIG. 11( a), the column of beamlets 1110-7, 1110-8 and 1110-9 is always transmitted through the horizontal steps 1102-0, 1102-1 and 1102-2 of the horizontal staircase element 1102. Thus, the column of beamlets 1110-7, 1110-8 and 1110-9 always has at least a minimum step height of 6 mm Additionally, some beamlets may experience path step heights greater than 6 mm, depending on whether the beamlets are also transmitted through either or all of the vertical steps 1104-0, 1104-1 and 1104-2 of the vertical staircase element 1104.

A beamlet 1110-7 only passes through the horizontal steps 1102-0, 1102-1 and 1102-2 and does not pass through either of the vertical steps 1104-1 or 1104-2 having a non-zero step height, and therefore has a step height of 6 mm (represented as the numeral “6” in FIG. 11( a)). A second beamlet 1110-8 passes through the horizontal steps 1102-0, 1102-1 and 1102-2 and additionally passes through the vertical step 1104-0, 1104-1, and therefore has a step height of 7 mm, which is the sum of the 6 mm from horizontal steps 1102-1 and 1102-2 and the 1 mm from vertical step 1104-0 and 1104-1 (represented as the numeral “7” in FIG. 11( a)). A third beamlet 1110-9 passes through the horizontal steps 1102-0, 1102-1 and 1102-2 and additionally passes through all of the vertical steps 1104-0, 1104-1 and 1104-2, and therefore has a step height of 8 mm, which is the sum of the 6 mm from horizontal steps 1102-1 and 1102-2 and the 2 mm from vertical steps 1104-0, 1104-1 and 1104-2 (represented as the numeral “8” in FIG. 11( a)).

Furthermore, as shown in FIG. 11( b), a column of beamlets 1110-4, 1110-5 and 1110-6 is always transmitted through the horizontal steps 1102-0 and 1102-1. Thus, the column of beamlets 1110-4, 1110-5 and 1110-6 always has at least a minimum step height of 3 mm. Additionally, some beamlets may experience optical path step heights greater than 3 mm, depending on whether the beamlets are also transmitted through either or all of the vertical steps 1104-0, 1104-1 and 1104-2 of the vertical staircase element 1104. A beamlet 1110-4 only passes through the horizontal steps 1102-0 and 1102-1 and does not pass through either of the vertical steps 1104-1 or 1104-2 having a non-zero step height, and therefore has a step height of 3 mm (represented as the numeral “3” in FIG. 11( a)). A beamlet 1110-5 passes through the horizontal steps 1102-0 and 1102-1 and additionally passes through the vertical step 1104-0 and 1104-1, and therefore has a step height of 4 mm, which is the sum of the 3 mm from horizontal step 1102-1 and the 1 mm from vertical step 1104-0 and 1104-1 (represented as the numeral “4” in FIG. 11( a)). A beamlet 1110-6 passes through the horizontal steps 1102-0 and 1102-1 and additionally passes through both of the vertical steps 1104-0, 1104-1 and 1104-2, and therefore has a step height of 5 mm, which is the sum of the 3 mm from horizontal step 1102-1 and the 2 mm from vertical steps 1104-0, 1104-1 and 1104-2 (represented as the numeral “5” in FIG. 11( a)).

Moreover, as shown in FIG. 11( b), a column of beamlets 1110-1, 1110-2 and 1110-3 is not transmitted through either of the horizontal steps 1102-1 or 1102-2, and is only transmitted through step 1102-0 which has a zero step height. Thus, the column of beamlets 1110-1, 1110-2 and 1110-3 may have an optical step height of 0 mm. Additionally, some beamlets may experience path step heights greater than 0 mm, depending on whether the beamlets are also transmitted through either or both of the vertical steps 1104-0, 1104-1 and 1104-2 of the vertical staircase element 1104. A beamlet 1110-1 does not pass through either of the vertical steps 1104-1 or 1104-2, and therefore has a step height of 0 mm (represented as the numeral “0” in FIG. 11( a)). A beamlet 1110-2 passes through the vertical step 1104-0 and 1104-1, and therefore has a step height of 1 mm (represented as the numeral “1” in FIG. 11( a)). A beamlet 1110-3 passes through both of the vertical steps 1104-0, 1104-1 and 1104-2, and therefore has a step height of 2 mm. (represented as the numeral “2” in FIG. 11( a)).

FIG. 11( a) depicts a view of a one-dimensional crossed staircase element with a projected plane representing the total step height of each beamlet passing through the one-dimensional crossed staircase element. In general, a principle behind the selection of the step heights of the horizontal staircase element 1102 and vertical staircase element 1104 in the one-dimensional crossed staircase element 1100 is based on the concept that each beamlet passing through the one-dimensional crossed staircase element 1100 should have a different step height from all of the other beamlets passing through the one-dimensional crossed staircase element 1100, in order to reduce or completely eliminate interference.

For example, if a vertical staircase element and horizontal staircase element were fabricated to have steps with the same step height (e.g., 1 mm), a problem would exist in that, by configuring both staircase elements and to have the same total step heights, more than one beamlet of laser light passing through the one-dimensional crossed despeckling element would have the same total step height (optical path difference). Thus, these beamlets with the same optical path difference would interfere with each other, creating undesirable results.

Therefore, as shown in FIG. 11( a), a principle governing the selection of step heights in the one-dimensional crossed staircase element 1100 according to aspects of the present invention is to select step heights to ensure that each beamlet passing through the one-dimensional crossed staircase element 1100 has a different step height from every other beamlet passing through the one-dimensional crossed staircase element 1100. As shown in FIG. 11( a), the vertical staircase element 1104 has three steps, wherein step 1104-0 has a step height of 0, and steps 1104-1 and 1104-2 each have a step height of 1 mm, for example (as indicated by the arrow labeled “1,” which indicates that light passing through step 1104-1 has a step height of 1 mm, and as indicated by the arrow labeled “2,” which indicates that light passing through step 1104-2 has a step height of 2 mm as a result of passing through both steps 1104-1 and 1104-2). To prevent interference, the step heights of the horizontal staircase element 1102, which also has three steps, are selected so that step 1102-0 has a step height of 0, and steps 1102-1 and 1102-2 each have a step height of 3 mm (as indicated by the arrows labeled “3” and “6”, respectively). As a beamlet passes through the steps in the horizontal staircase element 1102 and/or the steps in the vertical staircase element 1104, the beamlet experiences a total step height according to a sum of the steps that it passes through, as indicated by the projected plane 1150 shown in FIG. 11( a). For example, the beamlet which only passes through step 1102-0 (step height=0) and step 1104-0 (step height=0) has a total step height of 0 (0+0=0), as indicated by the box “0” in the upper left-hand corner of the projected plane 1150. The beamlet which passes through step 1102-1 and step 1104-2 has a total step height of 5(3+2=5), as indicated by the box “5” in the bottom center of the projected plane 1150. As indicated in the projected plane 1150, by selecting the step heights in this fashion, it can be guaranteed that no beamlets experience the same optical step heights as each other, and thus, interference is prevented. It should be understood that this description is exemplary only, and the steps are not limited to being set in units of 1 mm, and it should be further understood that the step heights are not limited to being selected in this fashion, and may instead be selected in various other fashions which also reduce interference.

FIG. 12 illustrates a one-dimensional crossed despeckling array 1300 according to a ninth embodiment. As shown in FIG. 12, the one-dimensional crossed despeckling array 1300 (also referred to herein as “array 1300”) shown in FIG. 12 is similar to the one-dimensional crossed staircase element 1100 shown in FIGS. 11( a)-11(c), except that the array 1300 includes a plurality of the horizontal staircase elements. In particular, the array 1300 includes four horizontal staircase elements 1302-1, 1302-2, 1302-3, and 1302-4 instead of one horizontal staircase element, each of the horizontal staircase elements 1302-1, 1302-2, 1302-3, and 1302-4 include four steps, and the vertical staircase element 1304 includes four steps, instead of the three steps included in the horizontal and vertical staircase elements 1102 and 1104. Also, another difference between the array 1300 and the one-dimensional crossed staircase element 1100 is that, unlike the horizontal staircase element 1102 which has one step having a step height of “0”, each of the steps of each of the horizontal staircase elements 1302-1, 1302-2, 1302-3 and 1302-4 have a non-zero step height. The reason for this configuration is to provide additional structural support so as to strengthen each of the horizontal staircase elements 1302-1, 1302-2, 1302-3 and 1302-4. The array 1300 can be formed in various ways known to those skilled in the art, and can be formed in substantially similar ways as those mentioned above with respect to the other embodiments. Furthermore, it is understood that arrays according to other aspects of the present invention can include more or less than four horizontal staircase elements and one vertical staircase element, and any combination is possible.

FIG. 13 illustrates an M× N laser diode (LD) module 1400 for laser displays according to a tenth embodiment of the present invention. As shown in FIG. 13, the laser module 1400 includes a housing 1410 which houses the array 1300 shown in FIG. 12 along with additional components. More specifically, the laser module 1400 includes an M×N (e.g., 4×2) grid of laser diodes 1402 (e.g., for a total of 8 laser diodes) to generate and transmit coherent laser light to the array 1300. The laser diodes 1402 may be the same as the laser diodes described above in other embodiments. The laser module further includes a first microlens 1404 disposed before the array 1300 to focus the light generated by the laser diodes 1402 into beamlets and transmit the beamlets towards the array 1300.

The array 1300 is configured to receive the beamlets transmitted from the first microlens 1404 and reduce speckle of the beamlets, as explained above with respect to the eighth and ninth embodiments. Similar to the other embodiments, the array 1300 is positioned near the foci of the beamlets transmitted from the first microlens array 1404 and reduces or completely eliminates diffraction of the beamlets. The beamlets with reduced speckle are transmitted through the array 1300 to a second microlens 1406, which relays the beamlets onto a field lens 1408. The field lens 1408 then focuses the beamlets with drastically reduced (or no) speckle onto an image plane. Additionally, various types of integrated circuits (not shown) and other components known to those skilled in the art may be included in the laser module 1400.

If a typical laser diode outputting approximately 0.25 W of output power is used and the laser module 1400 uses eight laser diodes 1402 arranged in a 4×2 grid pattern, the laser module 1400 can achieve nearly 2 W total output power or less considering the coupling loss between the laser diode and the collimator and losses by reflection from optics, which is more than powerful enough to create a laser annealing system and a laser display unit. Furthermore, since the laser module 1400 uses the array 1300, the laser module 1400 outputs coherent laser light with remarkably reduced or no speckle.

Furthermore, it is understood that the laser module 1400 is not limited to using a 4×2 grid of laser diodes 1402, and may instead use any combination of laser diodes known to those skilled in the art. According to aspects of the present embodiment, the laser module 1400 may be used with an M×N grid of laser diodes 1402, where M and N are positive integers respectively representing a number of laser diodes arranged in columns and rows of the grid, and where M>N, N≧1, and M≧2 (e.g., the 4×2 grid described above). By setting M>N, the laser module 1400 may be designed so that the field lens 1408 does not need to be the same length in each direction, which simplifies the design of the field lens 1408. Furthermore, the laser module 1400 may be configured so that the beam shaping axis is perpendicular to the direction of the grid which has relatively more laser diodes 1402. Thus, in the case of an M×N grid where M>N, the beam shaping axis should be in the direction of and correspond to the N laser diodes (as shown, for example, in FIGS. 8( a) and 8(b), where the beam shaping axis corresponds to the fast axis of 8(b)). However, it is understood that in the grid of laser diodes 1402 used in the array 1400, M need not be different from N; for example, in the grid 1402 M may be equal to N (e.g., yielding a 3×3 grid), in which case the field lens 1408 is configured to be the same length in each direction. It is further understood that the laser diodes 1402 are not limited to being arranged in a rectangular or square grid, and may alternatively be arranged in various other configurations as well (e.g., a triangular shape). Moreover, the laser module 1400 may be combined or modified in accordance with any of the other embodiments described above, as understood by those skilled in the art.

FIG. 14 depicts a two-dimensional staircase element 1500 according to an eleventh embodiment of the present invention. The main difference between the two-dimensional staircase element 1500 shown in FIG. 14 and the one-dimensional crossed staircase element 1100 shown in FIGS. 11( a)-11(c) is that the fabrication process to make the two-dimensional staircase element 1500 differs from the fabrication process to make the one-dimensional crossed staircase element 1100, even though both staircase elements 1500 and 1100 achieve substantially the same superior despeckling results. Specifically, the two-dimensional staircase element 1500 is formed by a single piece of transparent material and has a common flat entrance surface 1504 on one end, from which each of the steps 1502 protrude (similar in appearance to buildings protruding out from a common piece of land), whereas the one-dimensional crossed staircase element 1100 is formed by separately forming the horizontal staircase element 1102 and vertical staircase element 1104 and then adhering the horizontal staircase element 1102 and vertical staircase element 1104 together so that the steps protrude out in different directions from a middle area as in, e.g., FIG. 11( a), using any suitable adhesive material known to those skilled in the art.

As shown in FIG. 14, the two-dimensional staircase element 1500 includes a series of steps 1502 oriented parallel to each other. Each of the steps 1502 has a unique (different) step height as compared to each of the other steps 1502, to ensure that the beamlets passing through each of the steps 1502 do not interfere with each other, for the same reasons as described above with respect to the other embodiments. Thus, as exemplarily illustrated in FIG. 14, step 1502-0 has a step height of 0, step 1502-1 has a step height of 1, step 1502-2 has a step height of 2, and so forth. The step heights can be any value known to those skilled in the art, similar to the other embodiments of the present invention.

The two-dimensional staircase element 1500 functions in a similar fashion to the one-dimensional crossed staircase element 1100, and achieves similarly beneficial despeckling results.

FIGS. 15( a) and 15(b) depict the two-dimensional staircase element 1500 shown in FIG. 14 in use, viewed from a top perspective and a side perspective, respectively. In particular, FIG. 15( a) illustrates the two-dimensional staircase element 1500 viewed from a top perspective along a fast axis, and FIG. 15( b) illustrates the two-dimensional staircase element 1500 viewed from a side perspective along a slow axis.

As shown in FIG. 15( b), laser light 1510 passes through a two-dimensional microlens array 1514 (a fly's eye lens array) and is transmitted to the two-dimensional staircase element 1500. The two-dimensional microlens array 1514 is similar to the one-dimensional crossed microlens array 1114 (FIGS. 11( b), 11(c)), except that the two-dimensional microlens array 1514 is formed out of a single piece of material and has a common single surface from which the microlenses protrude, instead of being formed as two separate microlens arrays and then combined, like the one-dimensional crossed microlens array 1514. Collectively, the “two-dimensional microlens array 1514” and the “two-dimensional staircase element 1500” may also be referred to as a “two-dimensional despeckling unit.” The laser light 1510 can be generated by, for example, a laser diode as described with respect to other embodiments of the invention. The two-dimensional crossed microlens array 1514 includes a microlens arrays 1516 which is configured to focus the laser light 1510 along the optical axis.

The two-dimensional despeckling unit shown in FIGS. 15( a) and 15(b) functions in a similar way and achieves similar beneficial despeckling results as the one-dimensional crossed despeckling unit shown in FIGS. 11( b) and 11(c), and a detailed description of the functioning of the two-dimensional despeckling unit is therefore omitted.

In the two-dimensional despeckling unit shown in FIGS. 15( a) and 15(b), the two-dimensional staircase element 1500 is not limited to being used with only the two-dimensional microlens array 1514, and may alternatively be used with the one-dimensional crossed microlens array 1114. Similarly, the one-dimensional crossed staircase element 1100 is not limited to being used with only the one-dimensional crossed microlens array 1114, and may alternatively be used with the two-dimensional crossed microlens array 1514.

Furthermore, any of the components described above with respect to the two-dimensional despeckling unit shown in FIGS. 15( a)-15(b) may be combined or modified in accordance with any other embodiments described, as would be apparent to those skilled in the art. For example, the laser module 1400 (FIG. 13) could implement the two-dimensional staircase element 1500 alternatively to the one-dimensional crossed staircase element 1100.

The foregoing description illustrates and describes embodiments of the present invention. However, the disclosure shows and describes only the preferred embodiments of the invention, but it is to be understood that the invention is capable of use in various other combinations, modifications, and environments. Also, the invention is capable of change or modification, within the scope of the inventive concept, as expressed herein, that is commensurate with the above teachings and the skill or knowledge of one skilled in the relevant art. For example, one or more elements of each embodiment may be omitted or incorporated into the other embodiments.

The foregoing implementations and embodiments of the invention have been presented for purposes of non-limiting illustration and description. Although the present invention has been described herein with reference to particular structures, materials and embodiments, the present invention is not intended to be limited to the particular features and details disclosed herein. Rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims. Especially the present invention extends to all functionally equivalent reflective system in which either or all of the transmissive components such as fly's eye lens arrays and staircase element is made of reflective material. The descriptions provided herein are not exhaustive and do not limit the invention to the precise forms disclosed. The foregoing embodiment examples have been provided merely for purposes of explanation and are in no way to be construed as limiting the scope of the present invention. The words that have been used herein are words of description and illustration, rather than words of limitation. The present teachings can readily be realized and applied to other types of apparatuses. Further, modifications and variations, within the purview, scope and spirit of the appended claims and their equivalents, as presently stated and as amended hereafter, are possible in light of the above teachings or may be acquired from practicing the invention. Furthermore, although elements of the invention may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated. Alternative structures discussed for the purpose of highlighting the invention's advantages do not constitute prior art unless expressly so identified. No one or more features of the present invention are necessary or critical unless otherwise specified. 

1. A despeckle unit, comprising: a first transparent element comprising a plurality of microlenses to receive collimated light having a coherence length and to output a beamlet from each of the microlenses; and a second transparent element comprising a plurality of steps having a one-to-one correspondence with the plurality of microlenses, wherein each of the steps receives one of the beamlets and outputs the beamlet to an image plane, where a height of each step of at least two of the steps is configured to produce an optical path difference of the beamlet longer than the coherence length, wherein the second transparent element is disposed approximately at a foci of the beamlets output from the first transparent element.
 2. The despeckle unit according to claim 1, wherein the collimated light comprises pulsed laser beams driven by a pulse of less than 10 nanoseconds.
 3. The despeckle unit according to claim 2, wherein the pulse reduces a coherence of the laser beam and the steps are configured to further reduce the coherence of the laser beam not reduced by the pulse, to substantially despeckle the pulsed laser beam.
 4. The despeckle unit according to claim 1, wherein the at least two of the steps are configured as a one-dimensional staircase and the microlenses are configured as a one-dimensional array of microlenses.
 5. The despeckle unit according to claim 1, wherein: the collimated light is linearly polarized with a polarization direction; the second transparent element comprises at least one physical step, comprising: an optical wave plate disposed on a first portion of the at least one physical step which is configured to change the polarization direction of the collimated light, and a second portion of the at least one physical step which does not include the optical wave plate; and the optical waveplate and the second portion each comprise one of the steps having the one-to-one correspondence with the microlenses.
 6. The despeckle unit according to claim 1, wherein: the collimated light is linearly polarized with a polarization direction; the second transparent element comprises at least one physical step, comprising: a first optical wave plate disposed on a first portion of the at least one physical step which is configured to change the linearly polarized light to right circular polarized light, and a second optical wave plated disposed on a second portion of the at least one physical step which is configured to change the linearly polarized light to left circular polarized light; and the first optical wave plate and the second optical wave plate each comprise one of the steps having the one-to-one correspondence with the microlenses.
 7. A despeckle unit, comprising: a first transparent element comprising a plurality of microlenses to receive collimated light having a coherence length and to output a beamlet from each of the microlenses; and a second transparent element comprising a plurality of steps having a one-to-one correspondence with the plurality of microlenses, wherein each of the steps receives one of the beamlets and outputs the beamlet to an image plane, wherein a height of each step of at least two of the steps is configured to produce an optical path difference of the collimated light longer than the coherence length, wherein the second transparent element is disposed in a location relative to the first transparent element such that edges of the second transparent element parallel to an optical path of the beamlets exiting the second transparent element do not diffract the beamlets.
 8. The despeckle unit according to claim 7, wherein the collimated light comprises pulsed laser beams driven by a pulse of less than 10 nanoseconds.
 9. A despeckling laser unit to despeckle a laser beam, comprising: a laser generating unit to generate a pulsed laser beam having a coherence length; a first transparent element comprising a plurality of microlenses to receive the pulsed laser beam and to output a beamlet from each of the microlenses; and a second transparent element comprising a plurality of steps corresponding to the plurality of microlenses, wherein each of the steps receives one of the beamlets and outputs the beamlet to an image plane, wherein a height of each step of at least two of the steps is configured to produce an optical path difference of the pulsed laser beam longer than the coherence length; wherein the second transparent element is disposed approximately at the foci of the beamlets output from the first transparent element.
 10. The despeckling laser unit of claim 9, wherein the pulsed laser beams are driven by a pulse of less than 10 nanoseconds.
 11. The despeckling laser unit of claim 9, further comprising: a collimator disposed between the laser generating unit and the first transparent element to receive the pulsed laser beam and output a collimated laser beam; and a field lens to receive the beamlets output from the second transparent element and focus the received beamlets on the image plane.
 12. The despeckle unit according to claim 9, wherein the at least two of the steps are configured as a one-dimensional staircase and the microlenses are configured as a one-dimensional array of microlenses.
 13. The despeckle unit according to claim 9, wherein: the pulsed laser beam is linearly polarized with a polarization direction; the second transparent element comprises at least one physical step, comprising: an optical wave plate disposed on a first portion of the at least one physical step which is configured to change the polarization direction of the pulsed laser beam, and a second portion of the at least one physical step which does not include the optical wave plate; and the optical waveplate and the second portion each comprise one of the steps having the one-to-one correspondence with the microlenses.
 14. The despeckle unit according to claim 9, wherein: the pulsed laser beam is linearly polarized with a polarization direction; the second transparent element comprises at least one physical step, comprising: a first optical wave plate disposed on a first portion of the at least one physical step which is configured to change the linearly polarized light to right circular polarized light, and a second optical wave plated disposed on a second portion of the at least one physical step which is configured to change the linearly polarized light to left circular polarized light; and the first optical wave plate and the second optical wave plate each comprise one of the steps having the one-to-one correspondence with the microlenses.
 15. The despeckling laser unit of claim 9, further comprising a third transparent element comprising another plurality of microlenses disposed after the second transparent element and corresponding to the plurality of microlenses in one-to-one correspondence with the plurality of microlenses.
 16. A despeckling laser array, comprising: a plurality of the despeckling laser units according to claim 9; and a single field lens to focus the beamlets output from each of the plurality of despeckling laser units onto the image plane.
 17. A despeckling laser assembly, comprising: the despeckling laser array according to claim 16; a base plate to support the despeckling laser array; a circuit board attached to one end of the despeckling laser array; and at least one driver integrated circuit mounted on the circuit board to drive the laser generating units of the despeckling laser array.
 18. An annealing system to anneal a substrate, comprising: a plurality of the despeckling laser assemblies according to claim 17 disposed above a front surface of the substrate, such that each of the despeckling laser assemblies is configured to focus the beamlets on the substrate, wherein each of the despeckling laser assemblies is movable to enable the annealing system to anneal the front surface of the substrate.
 19. The annealing system of claim 18, wherein the substrate comprises amorphous silicon for organic LED displays.
 20. A one-dimensional crossed despeckling unit, comprising: a first transparent element comprising: a first surface having a first plurality of first microlenses to receive collimated light having a coherence length, and output a first plurality of first beamlets corresponding to the first plurality of microlenses, and a second surface having a second plurality of second microlenses to receive the collimated light, and output a second plurality of second beamlets corresponding to the second plurality of microlenses; and a second transparent element comprising: a first plurality of first steps oriented such that at least one of the first steps corresponds to at least one of the first beamlets; and a second plurality of second steps oriented such that at least one of the second steps corresponds to at least one of the second beamlets, wherein: a height of each step of at least two steps from among the first steps and the second steps is configured to produce an optical path difference of the pulsed laser beam longer than the coherence length, and the second transparent element is disposed approximately at a foci of the first beamlets and the second beamlets output from the first transparent element.
 21. The one-dimensional crossed despeckle unit according to claim 20, wherein the collimated light comprises pulsed laser beams driven by a pulse of less than 10 nanoseconds.
 22. The one-dimensional crossed despeckle unit according to claim 21, wherein the pulse reduces a coherence of the laser beam and the steps are configured to further reduce the coherence of the laser beam not reduced by the pulse, to substantially despeckle the pulsed laser beam.
 23. The one-dimensional crossed despeckling unit according to claim 20, wherein the first transparent element comprises a one-dimensional crossed microlens array, and the second transparent element comprises a combination of a first staircase element having the first plurality of steps and a second staircase element having the second plurality of steps.
 24. The one-dimensional crossed despeckling unit according to claim 23, wherein the first steps each have the same first height, the second steps each have the same second height, and the first height is different from the second height.
 25. The one-dimensional crossed despeckling unit according to claim 24, wherein the first staircase is provided in plural.
 26. A laser module, comprising: a housing; a plurality of laser diodes disposed at one end of the housing to generate respective pulsed laser beams having respective coherence lengths; a first transparent element, disposed after the plurality of laser diodes in the direction of travel of the laser beams, comprising a plurality of microlenses to receive the pulsed laser beams and to output a beamlet from each of the microlenses; and the one-dimensional crossed despeckling unit according to claim 25 disposed after the first transparent element in the direction of travel of the laser beams, wherein each of the first staircases corresponds to at least one of the laser diodes.
 27. The laser module of claim 26, wherein: the laser diodes are arranged in an M×N grid, where M an N are positive integers respectively representing a number of laser diodes in columns and rows of the grid; the plurality of first staircases comprise M first staircases, and each one of the M first staircases corresponds to a respective one of the M columns.
 28. The laser module of claim 27, wherein: M>N, N≧1, M≧2; a beam shaping axis is arranged in a direction corresponding to the N laser diodes; and each one of the M×N laser diodes generates approximately 0.2 watts (W) of output power.
 29. A method to despeckle a laser beam, comprising: generating a pulsed laser beam having a coherence length; transmitting the pulsed laser beam through a first transparent element comprising a plurality of microlenses so that the pulsed laser beam is output as a beamlet from each of the microlenses; and transmitting each one of the beamlets through a respective step included in a second transparent element comprising a plurality of the steps having a one-to-one correspondence with the plurality of microlenses, to an image plane, wherein a height of each step of at least two of the steps is configured to produce an optical path difference of the beamlets longer than the coherence length, wherein the second transparent element is disposed approximately at the foci of the beamlets output from the first transparent element.
 30. The method according to claim 29, wherein the generating of the pulsed laser beam comprises driving the pulsed laser beam using a pulse of less than 10 nanoseconds.
 31. The method according to claim 29, further comprising: collimating the generated pulsed laser beam and outputting the collimated generated pulsed laser beam to the first transparent element; and focusing the received beamlets transmitted through the respective steps by the second transparent element on the image plane using a field lens.
 32. The method according to claim 29, wherein the at least two of the steps are configured as a one-dimensional staircase and the microlenses are configured as a one-dimensional array of microlenses.
 33. The method according to claim 29, further comprising: polarizing the pulsed laser beam with a linear polarization having a polarization direction; and changing the polarization direction of the pulsed laser beam by passing the pulsed laser beam through an optical wave plate comprising one of the steps of the second transparent element.
 34. The method according to claim 29, further comprising: polarizing the pulsed laser beam with a linear polarization; and changing the linear polarization of the pulsed laser beam to right and left circular polarization by passing the pulsed laser beam through corresponding first and second optical wave plates each comprising one of the steps of the second transparent element.
 35. The method according to claim 29, further comprising transmitting each one of the beamlets output from the second transparent element through a third transparent element comprising another plurality of microlenses in one-to-one correspondence with the plurality of the microlenses.
 36. A two-dimensional despeckling unit, comprising: a first transparent element comprising: a surface having a plurality of microlenses to receive collimated light having a coherence length from a pulsed laser beam, each of the microlenses configured to output a beamlet which is shaped in two-dimensions; and a second transparent element comprising: a light incident surface forming a two-dimensional area comprising two first boundaries and two second boundaries perpendicular to and connecting the two first boundaries; and a plurality of steps protruding out from the light incident surface and arranged in rows, wherein the steps in each row are configured to increase in height along a first direction parallel to the first boundaries, and the rows increase in height along a second direction parallel to the second boundaries, each of the steps having a different height from each other, and each of the steps being configured to receive a corresponding one of the beamlets; wherein: the height of each step is configured to produce an optical path difference longer than the coherence length, and the light incident surface is disposed approximately at a foci of the beamlets output from the first transparent element. 